The present invention concerns a liquid unsaturated polyester resin, as well as its manufacturing process, making it possible to create preimpregnated materials (called "prepregs") which possess an accelerated maturation rate; i.e., which can can be molded very soon after their manufacture. Moreover, the stability of the preimpregnated materials prepared from these resins remains excellent on storage and also permits molding after a time exceeding several months.
A preimpregnated polyester material is a compound consisting mainly of a mixture of unsaturated polyester resin, a filler, a coloring matter, an unmolding agent, glass fibers, a cross-linking agent and a thickening agent. This composition, which initially is liquid, cannot be handled readily enough in order to be molded as is. Prior to molding, it must be subjected to a maturation process for the purpose of increasing its viscosity. One of the known maturation processes consists of producing a sandwich material composed of the composition of the preimpregnated material, which must preserve its polymerization and cross-linking properties, between two films of polyethylene. The thickening agent brings about an increase in viscosity which after a certain length of time makes it possible for the composition to be handled for molding.
After maturing and, in the case of the preimpregnated material cited above, the elimination of the polyethylene films, the preimpregnated material can be molded according to the usual hot compression molding processes, with the cross linkage being obtained by decomposition of the cross-linking agent, which as a rule is an organic peroxide.
The thickening agent introduced for the maturation process brings about an increase in the viscosity of the preimpregnated composition, which must be such that the viscosity is sufficient in order to allow molding. If the viscosity is insufficient, large losses of composition are observed on the molds, and if the viscosity is too high, the flow of the preimpregnated compositions into and in the mold is poor, causing a faulty appearance and poor mechanical properties of the molded piece. For a good utilization of the preimpregnated material, it is recommended that the composition for the preimpregnated material, in the absence of glass fibers, should reach a minimum viscosity for molding of about 5.times.10.sup.5 poise in a minimum time and a maximum viscosity for molding of about 2.times.10.sup.6 poise in a maximum time. These values are given by way of illustration only and must be measured on a composition for preimpregnated material which does not contain glass fibers, since the latter make any valid measurement of the viscosity impossible.
With the unsaturated polyester resins which are known, the maturation time at ambient temperature making it possible to reach the minimum viscosity for molding exceeds 48 hours, and this presents the disadvantage of necessitating a large stockpiling of preimpregnated composition, in the process of maturing, for the manufacture of molded objects of unsaturated polyesters.
In order to shorten these maturing periods, it is, for instance, possible to increase the quantities of thickening agent, but in that case, if the maturing periods are effectively shortened, the preimpregnated composition reaches its maximum viscosity for molding all the more quickly, and this runs the risk of seriously limiting the periods of time during which it is possible to mold the composition.